Many apologies for the lateness of this installment. In Part I and Part 2 we learned a bit of backstory and what set the new stage for the equine's development. In this Part 2, we'll delve into more detail about what makes a horse a horse.
Now it may seem a bit odd that we're discussing equine evolution in an anatomy series. But the fact is when we discover the why the horse is built like a horse, we can adopt modes of thinking that don't compromise our fidelity to our subject. Understanding his past makes it harder to be so arbitrary in our creative decisions, and we begin to adopt an advocate point of view for this animal we profess to love. Our work gains greater authority and responsibility, and that bodes well for our unfurling body of work.
So let's get to it!...
Now it may seem a bit odd that we're discussing equine evolution in an anatomy series. But the fact is when we discover the why the horse is built like a horse, we can adopt modes of thinking that don't compromise our fidelity to our subject. Understanding his past makes it harder to be so arbitrary in our creative decisions, and we begin to adopt an advocate point of view for this animal we profess to love. Our work gains greater authority and responsibility, and that bodes well for our unfurling body of work.
So let's get to it!...
Now For Some Detail…
Little eohippus began life in the dark, dense forests of prehistory, nibbling on shoots and leaves, and scampering around like big bunnies. Over time, the forests gave way to grasses as the climate changed, but in order for the grazing lines to adapt to a life on the plains, a series of specific adaptations had to occur, starting about 20 million years ago.
The early equids were browsers, eating the abundant fruits, shoots, and leaves which are greater sources of nutrients than grasses. But to exploit the grasslands, he had to develop a digestive system that could process grasses rather than forest vegetation. In many ways, Equus truly “is what he ate” since many of his later adaptations from speed, size, and intelligence, can be directly linked to the change in his diet. But digesting grasses is no easy task, being a comparatively poor food source relative to forest plants. It also has cellulouse, the complex sugar in fibrous plants. Ultimately, grasses cannot be broken down by a mammal without the aid of gut bacteria, which breaks down this cellulouse into volatile fatty acids the animal can process. Yet it’s a time-consuming method that requires a chamber for this food matter to be stored so the bacteria can work their magic. There are two different ways this relationship has been expressed in ungulates. One is ruminant digestion (or pre-gastric fermentation) and the other is cecal digestion (or post-gastric fermentation). Artiodactyls evolved ruminant digestion first, entailing four stomach chambers (though camels aren’t considered “classic ruminants” since they have three stomach chambers rather than four, though it functions similarly). The first two chambers, the rumen and reticulum are where bacterial fermentation occurs. Then the food material is regurgitated and chewed again, otherwise known as “chewing the cud." Upon being swallowed again, the food matter passes through a special opening into the last two chambers of the stomach, the omasum and abomasums, where further digestion takes place. Pound per pound, rumination is still the most efficient means for extracting nutrients from grasses largely because food matter ferments for the longest period of time. It takes about 70-90 hours for food to pass through a cow, for example. However, non-ruminant Artiodactyls do exist such as pigs, peccaries, and hippos.
In contrast, Perissodactyls evolved cecal digestion. This entails digestion in the cecum (equivalent to the human appendix), which grew to enormous size to provide the fermentation chamber. Food is chewed and travels to a comparatively small stomach, which then makes it into a slurry with digestive juices, and then passes the materials into the cecum for bacterial fermentation. Then from the cecum, the materials travel to the intestines for further digestion. Because cecal digestion is a more effective digestive strategy in animals under 5 kg, it’s believed Perissodactyla may have adopted it while still small during the Paleocene. However, cecal digestion extracts about 30% less energy from the same food material as does ruminant digestion, but the benefit is that it takes far less time to digest the food matter, only about 48 hours. This allows equines to flourish in niches where few other animals could survive or compete, particularly ruminant Artiodactyls. Biological data reveals that wild equines usually target the worst, lowest quality and highest fiber roughage they encounter. For example, while ruminates may eat just the leaves, equines will eat the stems left behind, as observed between Plains zebras and Gnus. In North America, feral horses and cattle compete for the same forage, but horses utilize territories not exploited by cattle, such as those far away from water sources (especially in winter) and at higher elevations. (An interesting factoid is that feral horses don’t compete with the Pronghorn, a native ungulate, since the Pronghorn eats mostly shrubs, herbs, and woody forage.) Indeed, many wild equines thrive in areas characterized by such poor quality vegetation that ruminants seem to avoid the area altogether. For example, wild asses and Takhi thrive in regions other large grazers deliberately avoid. Studies have also supported this data by showing that unless a certain level of fiber is provided by the habitat, a ruminant simply cannot support itself and will starve. The answer lies in the time factor. A ruminant can only process a limited amount of food in its system in a fixed period, which is a very long time. However, an equine’s response to poor vegetation is to simply eat more since its form of digestion can process nutrients much faster. So while per unit of energy a ruminant is more efficient per unit of time, a horse can extract far more energy from grasses than a ruminant. And we all know how horses love to eat! Nevertheless, his gut became large and heavy, so while the back of Equus appears hollow, due to his convex dorsal processes, his spine is really built in a soft arch to best support his heavy gut.
In addition, all this extra time a horse spends grazing amounts to about 15 hrs a day, in comparison to a cow who spends only about 8-10 hours a day grazing. All this “head down” time can translate into a greater vulnerability to predation, which has some interesting implications with how the equine evolved. For example, unlike ruminants, the horse’s digestive system allows him to “eat on the go," one moment munching at lunch and the next avoiding becoming one. This digestive design is also ideal for long journeys to search for new grazing areas or migration into new regions. So while cecal digestion is considered rather primitive, it could actually be considered a strange evolutionary benefit for Equus.
But with his cecal digestion, he also had to have teeth that could chew grass in sandy, gritty soil without quickly milling them down to nubs. Unlike fruits, shoots, and leaves, which either depend on being eaten to propagate or can suffer the loss of some material without sacrificing itself, the blades of grass are the plant itself. And plants enduring heavy herbivore depletion usually evolve various defenses. In grasses, this defense entailed infusing sturdy silica particles, or phytoliths, into the cell walls. Essentially, the nutrients in grass are locked in this silica skeleton, similar to glass powder, which lends shape and stiffness to the blade of grass, even when it’s dead. But to pulverize this silica skeleton to release the nutrients, lots of chewing is involved, but which rapidly wears down teeth because of the abrasive silica particles. In response, the horse lost his short, gently cusped bunodont teeth in lieu of a design that could withstand and exploit this abrasion. The gentle dentin cusps became elongated into long prongs sheathed in hard enamel, which then turned into slicing ridges. Hard cementum, once vestigial on the old bunodont teeth, encapsulated the entire tooth and filled in any spaces, particularly between the dentin prongs. Eventually, he developed continually erupting teeth (hypselodont), long crowned teeth (hysodont teeth) with long ridges made of blended dentin cusps (lophodont teeth) coated in tough enamel and encapsulated with hard cementum. So by altering the structure of the tooth and adding layers of materials of differing hardness (and therefore different rates of wear), nature created a grinding surface that responded to the abrasiveness of grass like a self-sharpening blade. This structure could withstand the silica particles and more efficiently turn grass into the necessary consistency of milled, fine cornmeal, increasing digestive efficiency (parts of the silica skeleton that aren’t pulverized won’t digest because gut bacteria cannot penetrate the silica shell).
The fine texture of chewed grasses also avoids deadly blockages in the horse’s digestive system, which is already vulnerable to obstruction with its various bottlenecks. Any grass bits longer than 3/8-1/2 inches (particularly 1/2-1 inches) can be lethal to a horse by causing obstructions and colic, which is why old horses, who often have dental problems, typically “quid” (chewing and sucking out the flavor of hay, then spitting out the fibrous wad rather than swallowing it). They realize the potential danger. The horse doesn’t chew his cud nor can he vomit, so whatever he swallows goes through his system on a one-way trip. So his teeth make his food both safe to eat and nutrient-rich for his body…a big responsibility for such unassuming anatomy!
His teeth also changed orientation in his skull, forming into front incisors for nipping off grass in the grazing position and developing rows of large grinders in the back. This resulted in a long diastema, which elongated his head below his eyes. For example, about 40 million years ago, with Mesohippus, this battery of grinding teeth was increased to six (the number in living horses today) on either side of the jaw, top and bottom, because his premolars adapted into grinders. And over time, the head of Equus would become longer and larger, pulling the teeth out from under his eye orbit and deepening his jaws to make room for these new, bigger, high-crowned teeth and the larger chewing muscles required to activate them. Another interesting benefit of a long head with a high-placed eye is that he could now spend the necessary long hours to graze, yet still have eyes reasonably high enough to scan the grassline for predators. Furthermore, the mechanics of his mandible joint changed to discourage for and aft motion, but to favor side-to-side rotary chewing, like a shearing motion still distinctive today.
Another adaptation for grazing were his lips, which in the modern horse have an almost prehensile quality, unusually sensitive, and adept enough for selecting choice bits of food. This design was produced by a shortening of his nasal bones that permitted a fleshier and more flexible muzzle advantageous for selective grazing. Actually, some extinct lineages had skull structures that implied very prehensile lips, such as those of the tapir. The shortened nasal bones also benefited his nostrils by allowing them to enlarge and become fleshier, enabling greater intakes of air. His muzzle also grew in size to facilitate the heavy breathing for running over distance and to accommodate the chewing of lots of grass.
Adaptations for grazing also occurred in his cervical spine, largely being for length to get those teeth down to the grass. Very early on, the horse’s cervical column pirated the first thoracic vertebra, making it more like a neck bone, rendering it functionally part of the neck, increasing length and flexibility. Also, his Sternomandibularis muscle, the “hugging muscle," developed for grazing so he could jerk his head back in that typical grazing, nipping motion with the incisors (in contrast to the possible snout-pushing motion of Hyracotherium).
A plains lifestyle also meant he could no longer rely on dodging about forest cover to escape predation. Rather, he had to increasingly rely on his fleetness of foot, in a straight line, for a sustained time over hard semi-level ground, to survive. Fortunately, Equidae was built for a running escape from the onset because of their narrow ribcages and scapulas on the sides of the torso, orientating them on the same plane whether running, rearing, or standing (in comparison, humans have a wide ribcage with scapulas situated on the back).
All the same, an important adaptation for running first occurred in his spine, which is still characteristic today. In Hyracotherium, the door for this necessary spinal change had already been opened, already differing this animal from related lines. How? Well, because his lumbar vertebrae were smaller and more condensed than other relations, even losing some vertebrae to establish the characteristic six lumbar in modern horses. The transverse processes of his lumbar were also more vertical and closer together, reducing their rotary capabilities. This established the horse, very early on, as primarily a natural “transverse” galloper in which a stride has a leading side (in contrast to a “rotary” galloper who runs with a cross-firing lead, like a lion). To improve the stability of the spine for a fleeing tactic on the plains, evolution began to straighten and stiffen it while further compressing the lumbar span to stabilize the coupling between the body and hindquarter. Over time, the lumbar vertebrae became even more rigidly constructed, with longer vertical transverse processes situated closer together and with a peculiar articulation between his last lumbar and sacrum. So rather than flexible lateral motion or rotation, this type of spine became increasingly limited to a coiling motion, the spinal structure characteristic of Equus today.
All these changes, started in Hyracotherium, were first seen in Parahippus, who possessed the first horse-like spine. This type of spine is very efficient for this new mode of escape, acting like a coil and spring mechanism specifically designed for acceleration and an enduring, rapid “cruising speed." And, indeed, for a large prey animal with a fermentative gut, who runs in a straight line, it’s far more efficient to have a stiff "pole" launched by that spring rather than a wiggly, wobbly pole. So by diverging from a dodging, flexible spine to the more rigid and straight posture built for speed in a straight path, Parahippus could literally outlast predators in the chase at high speeds, a survival tactic characteristic of Equus today. Nonetheless, the horse wasn’t the first to achieve this design since it was the camel family that has earned that distinction. The camel was the first to achieve the straightened, stiffened spine, and high withers for anchoring and a shortened lumbar span necessary for a running escape on the plains (this family was also the first to achieve the ruminant digestion and teeth necessary for feeding on grasses). Thusly, because the horse got a relatively slow start with a cursorial lifestyle, his back is still more flexible and arched than a camel’s or most Artiodactyl descendants.
His pelvis also changed by lengthening both ichium bones to increase leverage for the developing hamstring muscles that propelled him forwards, the Semitendinosus and Semimembranosus muscles (the "semis"). Not coincidentally, his first two tailbones were pirated by the sacrum, pulled forwards and made larger to functionally serve as the root for these enlarged hamstrings. And today, these muscles in the modern horse attach at the first two tailbones and the end of the sacrum and run down to the femur and tibia, becoming powerful motor muscles for propulsion. And somewhere during evolution, the Semitendinosus also developed a thick tendon in the middle of its muscle belly to amplify its forces and to become part of the Reciprocal Apparatus. So the dock on the modern horse really begins at the third tailbone and that bump sometimes seen on the tail head is really the Semitendinosus muscle. Additionally, the sacral spines on Parahippus were the first to slope backward, opposed by those of the lumbar that slope forwards. All of these changes together imply that this early horse may even have had a precursor system of the Reciprocal Apparatus.
Also in Parahippus, the horse began to increase the length of his withers to anchor the longer neck and larger head, perhaps also developing a crest. Withers didn’t develop in the browsing lines for there was no need, so their necks were round or tubular, like a dog. And started with Hyracotherium, and completed with his plains descendants, grazers also lost certain spines on their cervical vertebrae, implying that their necks began to rely on yellow ligaments for passive support rather than active muscle contraction.
Additional change occurred in his limbs, which needed modification for speed on the open plain. A common misconception maintains that the legs of early browsers became progressively longer first, making the animal increasingly larger as an inevitable outcome of equine evolution. But height and size only came into play after his digestive track, teeth, and, especially, his spine adapted to a cursorial lifestyle. The proportional differences between Hyracotherium (55 mya) and Mesohippus (40 mya), for example, were still relatively similar. In the lineages leading to Equus, the lateral digits became vestigial, leaving the 3rd digit to bear all the weight. The first digit to be lost was the first (or thumb), then the fifth (or pinky). Later the second (pointer finger) and the fourth (wedding finger) became vestigial toes or dewclaws, and now remain as the splint bones in the modern horse.
But it wasn’t until Parahippus (22 mya) that the legs themselves began to elongate by “telescoping” or lengthening the bones in his appendicular column, particularly those of his “hand” and “foot." This changed the proportions of his legs from those of his browsing relatives by making his upper limbs (scapula, humerus, and femur) comparatively shorter than his lower limbs. This is a typical characteristic in speed animals who require leg structures based on long levers with high fulcrums, or a short upper limb ratio to a longer lower limb ratio. So leverage became oriented about one-thirds up on the appendicular column (from the scapula to the elbow, or femoral joint to stifle) to act upon the remaining two-thirds (from the elbow to the toe, or from the stifle to toe). This means that with minimal effort by the upper limbs a great deal of leverage could happen at the toe, increasing stride length and therefore increasing speed, allowing him to “eat up the ground” with relative ease. In response, his limb structure simplified, removing the muscles below the carpus and tarsus, making the distal limbs passive levers activated by tendinous servos whose muscle bellies are located high on the limb (above the knee and hock). This reduced the weight of the distal limb, thereby further maximizing speed efficiency. For this reason the lower limbs of modern horses aren’t muscled. The "bone" of his legs also increased to allow a large, heavy herbivore have enough support, especially while escaping predators.
The plane of limb motion narrowed to eliminate inward or outward instability, too. For example, people can turn their palms upwards or downwards, known as “suppination of the manus," because the human radius isn’t fused to the ulna. Similarly, the browsing lines, the radius and ulna weren't fused but their movement was inhibited by a special design at the top of the radius. But in the grazing lines, the ulna became fused to the radius, totally preventing this “suppination of the manus” to keep the toe permanently pointed forwards. Likewise, the joints of his legs became structurally oriented on the same plane so that each time the horse articulated his legs, his toes would automatically be pointed forwards for flight. In the hindlimb, this resulted in an angled hock joint that keeps the toe relatively pointed forwards despite stifle flexion. The evolution of the equine leg also resulted in a cascade of check ligaments.
Another interesting change happened to the equine clavicle, or collarbone. In humans, the clavicle is connected by bony attachment, keeping the ribcage centered between the arms while also creating a bony attachment for the arms directly onto the torso (a relic from our primate ancestor who needed this design for climbing and swinging from branches). But like many large, hoofed animals with a suspension-based gait, the horse lost his clavicles during evolution, opting instead of the Shoulder Sling of muscle attachment. In the horse, this produced fluid, dynamic shoulder motion conducive for an agile, athletic long stride paired with shock absorption. Therefore, this large, speed animal could jump, pivot, stretch, and sprint at high speed without continually fracturing his collarbones. But, the loss of the collarbones forced his giant neck muscle, the Brachiocephalicus, to search for another attachment, ultimately finding the humerus, thereby linking his head directly to his arm and, therefore, to his entire foreleg.
Changes also occurred in his feet. The footing on the plains was quite different from that found on the forest floor, being abrasive and rough with hard surfaces and coarse plants. This heightened the need for tough horn and cornfied structures rather than soft footpads to carry a large herbivorel at speed over distance. His ancestors had a plantigrade stance, but as certain lines evolved, such as Hyracotherium, digitigrade stance developed. And in the lineage that produced the modern horse, this stance became unguligrade to increase speed and torque in a plains habitat. So during evolution, the feet of Hyracotherium's grazing descendants simplified, streamlined, and grew in size to serve the cursorial lifestyle, resulting in greater dependence on the middle toe, which lead to gradual atrophy or even loss of the other digits in some descendants. In the modern horse, the unguligrade stance found full development, causing the atrophied digits to disappear altogether and the digital toe pads (or central pads) became incorporated into his sole, forming the frog and digital cushion. His old distal metacarpal pads are believed to have become the ergots and his ancient proximal metacarpal and wrist pads may have become the chestnuts. Plus, the sensitive laminae of his foot are actually modified periosteum of the coffin bone, richly laden with Sharpey’s fibers, lashing the hoof capsule onto the once ancient middle toe. In all, the modern horse literally walks on tiptoe on his middle digit and on stilts telescoped from his “hands” and “feet." Remember the "LandStriders" from The Dark Crystal? Yeah, something like that. Interestingly enough though, Equus still has the capacity to recreate these vestigial side toes II and IV. Normally, these digits remain as the “splint bones," but sometimes a foal is born with these toes fully developed, “hoofies” and all. In fact, during fetal development, equine embryos still have three toes at six weeks, which diminish to the familiar design after about five months in the womb. Caesar was even said to ride a horse with “feet that were almost human, the hoofs being cleft like toes,” which soothsayers foretold as proof of Caesar’s destiny to rule the known world.
Predation was also a determining factor in equine evolution. Originally the Perissodactyla were the most plentiful and diverse hooved animals. Then the evolution of Artiodactyla blossomed, replacing the Perissodactyla as the dominant hooved animal. Indeed, even today the Artiodactyla are more plentiful and successful than the Perissodactyla, many of which are currently near extinction. Nevertheless, by the end of the Ogliocene, as early horses started to enter the plains, they initially competed quite well with the Artiodactyls and diversified quickly into large numbers during the Miocene.
But what's particularly interesting is how Artiodactyla may have primed the predator species on the plains, creating a habitat full of smart, fast, and efficient hunters. Predators had become more intelligent to outwit Artiodactyla prey long before the horse lineages showed up. It was actually a constant “cold war” of intelligence and wits between these animals that the early horses stumbled upon as they ventured onto the plains. So during these tentative first steps, early horses had to deal with packs of roving predators already well-equipped to make them an easy lunch. Hardly an optimistic beginning, yet this may have hastened and diversified equine evolution as those unsuitable were quickly removed from the gene pool. Also, the frontal lobes of Hyracotherium weren’t impressive, leading some to consider him rather dull-witted, but this “cold war” environment could have accelerated the development of equine brainpower since those simply too dim were quickly devoured. And as for the predators themselves, they were a diverse gaggle of looming danger. Since dinosaurs had been gone for some 10 million years before the Eocene, Hyracotherium didn’t have to worry about them gobbling him up. But on the other hand, he did have to worry about eight foot tall “terror birds” and the creodonts, an extinct group of carnivores who used an ambush and chase tactic to take down early camels, which they used on early horses. These creodonts would later evolve into modern cats and dogs during the Miocene. In fact, the Oligocene saw the beginnings of the Saber-Toothed Cat, an imposing predator, indeed. So the early horses certainly had their evolutionary work cut out for them to change from a browser lifestyle to a cursorial one…and they had to do it quickly.
Conclusion to Part VI, Evolution Part II
Fascinating stuff, huh? "You are what you eat" is literally true for the equine! It also suggests that all of these changes are necessary for his lifestyle, and therefore for his well-being. It also implies that there's very little fudge-factor in his biological underpinnings since he's such a delicately balanced biomechanical system based entirely on functional attributes. This has critical implications regarding many of the breeding trends we see today in some popular breeds, and thus in our creative decisions in the studio.
In the third part of "Evolution," we'll learn even more about his biology for more informed creative choices. The horse has undergone many fundamental changes through the eons, each one of them distinct to the equine. He's a unique animal—archaic, ancient, and full of surprises!
So until next time...may our thinking evolve, too!
"We must keep the goal for integrating new information ever in front of us."
~ Carole Mayne